In electrophotographic and electrostatic printing processes (collectively electrographic processes), an electrostatic image is formed on the surface of a photoreceptive element or dielectric element, respectively, with a toner. In electrostatic printing, a latent image is typically formed by (1) placing a charge image onto a dielectric element (typically the receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a charge image, (2) applying toner to the charge image, and (3) fixing the toned image. An example of this type of process is described in U.S. Pat. No. 5,262,259. In electrophotographic printing, also referred to as xerography, electrophotographic technology is used to produce images on a final image receptor, such as paper, film, or the like. Electrophotographic technology is incorporated into a wide range of equipment including photocopiers, laser printers, facsimile machines, and the like.
Electrophotography typically involves the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, and cleaning, and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential opposite in polarity to the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically a nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron, liquid toners are capable of producing very high-resolution toned images.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
Typical colored pigment particles previously used in toners are generally characterized in the classes of dyes, pigments and lakes. A dye is defined in Webster's dictionary as “a usually soluble substance for staining or coloring e.g. fabrics or hair.” Likewise, Webster's defines pigment as “dry coloring matter (especially an insoluble powder to be mixed with a liquid to produce paint etc).” A lake is defined as “A pigment formed by combining some coloring matter, usually by precipitation, with a metallic oxide or earth, esp. with aluminium hydrate; as, madder lake; Florentine lake; yellow lake, etc.” Lake pigments are conventionally prepared by the precipitation of a soluble organic dye on to an insoluble, inorganic, adsorptive substrate. The pigment is formed by the chemical reaction that occurs when a suitable reagent, such as alum, is added to an aqueous solution containing the dyestuff and, usually, one or more other chemicals, such as sodium or potassium carbonate.
Permanent pigments have been sought to provide lasting color in various applications. For example, U.S. Pat. No. 4,576,649 to Oliver, et. al. discloses preparation of permanent pigments from selected cationic dyes by precipitation from aqueous solution with complex heteropoly acids in the presence of selected amine color enhancing agents. These pigments are stated to be of interest in formulating high quality lithographic inks.
Colorants comprising bound dyes have been incorporated into toners for electrostatographic printing and copying processes. For example, U.S. Pat. No. 5,434,030 to Smith, et al. discloses a toner composition including a resin, a charge enhancing additive, an ion binding polymer, and at least one ionic dye complexed to the ion binding polymer. The ionic dye is a component that is different from the charge enhancing additive, and is stabilized and dispersed in the toner composition.
U.S. Pat. No. 5,766,269 to Berenguer, et. al. describes cationic dyes comprising at least one quaternary ammonium group or protonated or protonizable tertiary amino group and at least one chromophoric radical, wherein the chromophoric radical is the radical of an optionally (pre)reduced sulphur dye, its precursor containing a cationic group which is a secondary amino group of basic character. These dyes are stated to be combinable with an anionic desolubilizer to form a water insoluble dye. Examples of desolubilizers include, among other examples, anionic dyes. See column 15, lines 37–50.
U.S. Pat. No. 6,001,524 to Yoon discloses toner particles for color electrophotographic imaging applications. The particles are based on substantially amorphous polyesters that contain functionalities that are capable of being reacted with coloring reagents to form dyed polyester particles, which after further treatment are incorporated into toner compositions.